The telephone line testing principles described in this disclosure are based upon time domain reflectometry and are similar to the operation of radar. A signal is launched from a time domain reflectometer (TDR) connected to the line under test (LUT).
Time domain reflectometry is used for diagnosing problems in telephone and DSL circuits. A TDR operates by transmitting a signal on a transmission line and then monitoring the transmission line for any reflection of the signal. Reflections are caused by changes in the impedance along the transmission line. A change in impedance may indicate the presence of a fault. As a signal transmitted by a TDR reaches the impedance mismatch, a portion of the transmitted signal is reflected back to the TDR. Because the transmitted and reflected signals travel along the transmission line at a known speed of propagation, a precise location of the impedance mismatch may be determined by measuring the time at which the signal is transmitted and the time at which the reflected signal is received by the TDR.
The magnitude of the reflected signal is proportional to the magnitude of the impedance mismatch. The sign or polarity of the reflected pulse is determined by the direction of the change in impedance. For example, if the transmitted signal is positive and the impedance of the fault increases, then the reflected signal will be positive. A break in the line, for example, will result in strong positive reflected signal. If the transmitted signal is positive and the impedance at the fault decreases, then the reflected signal will be negative. A short in the line, for example, will produce a negative reflected pulse. Thus, the nature of the fault may be determined or inferred from analysis of the reflected waveforms.
The energy of the transmitted signal is dependent on the width of the signal. The larger the pulse width, the lower the frequency and the more energy is transmitted allowing the signal to travel further down the line. Accordingly, many currently available TDRs have a limited number of selectable pulse width settings. Each pulse setting produces pulses of substantially identical width.
Two types of TDR in use today are pulse TDR and step TDR. Pulse TDR, is commonly used in testing telecommunication lines. Pulse TDR provides an impulse wave shape to stimulate the LUT. Pulse TDR only provides a report of a differential response to impedance changes on the LUT. This differential response is typically adequate for detecting the end of the line, short circuits, or open circuits. Pulse TDR uses impulse which are pulse-shaped. The widths of the pulse-shaped impulses range from a few nanoseconds up to a few microseconds. Shorter impulse widths are used for short range testing (e.g. less than a few hundred feet) and longer impulse widths are used for longer range testing (e.g. thousands of feet). Pulse TDR is useful for approximating fault characteristics, but cannot measure line impedance and the exact nature of close-in faults. With pulse TDR there is no means of determining line impedance. Some line faults measured by pulse TDR result in complex waveforms shown on the screen that are difficult for the user to interpret. Thus, when a technician wants a better definition of the LUT they must use a second instrument such as a step TDR.
Step TDR is not commonly used in testing telecommunication lines due to high circuit complexity and sensitivity to damage from hazardous voltages found on the telecommunications lines. When Step TDR is performed, a step-shaped impulse is applied to the LUT. The step-shaped impulse starts with a very fast rising edge (e.g. a rise occurring in less than one nanosecond) and continues outputting a DC voltage on the LUT for up to a few microseconds. This technique results in an effective “traveling ohmmeter” as the step-shaped impulse propagates down the LUT. The fast rising edge and the following DC level are now tracked over time. As the step-shaped impulse encounters an impedance change, the reflected signal is measured as an offset to the nominal DC level, and provides a mechanism to report the impedance of the LUT inch by inch. This is much easier to interpret than pulse TDR and is capable of accurate measurement of faults on the LUT. Step TDR provides a direct impedance read out of the LUT over the range of interest, not possible with pulse (or differential) TDR methods. Step TDR is useful over shorter distances, typically up to several hundred meters depending on the quality of the LUT.
Certain step TDR devices have been provided for testing of telecommunications lines, such as for example a test sold by AEA under the trademark 20/20 TDR. These step TDR devices use a DC coupling method to the LUT. As a result, these conventional step TDR devices are not recommended on live tip/ring circuits due to likely damage from telephone line voltages. For this reason, the step TDR products currently on the market provide warnings and cautions concerning damage to these devices when used on phone lines and other sources of voltages. In addition, most step TDR instruments are sensitive to damage from voltages present on working telephone lines.